3D ion trap

With quadrupole CID all fragments are recorded in one experiment, while in the case of the 3D ion trap MS, MS and experiments are required to ob-... 

Quadmpole ion trap refers in general to a 3D ion trap instrument... 

Although the pharmaceutical industry has long recognized the conventional three-dimensional (3D) ion trap as a powerful tool for structural elucidation of metabolites,... 

The two-dimensional (2-D) or linear ion trap (LIT) emerged in the 2000s as an effective alternative to the 3-D trap. Before 1995, linear traps were used primarily as ion storage/transfer/ion-molecule reaction devices in combination with FTICR (Senko et al., 1997 Belov et al., 2001), TOF (Collings et al., 2001), 3D ion trap (Cha et al., 2000), and triple-quadrupole (Dolnikowski et al., 1988) mass spectrometers because LITs offer better ion storage efficiencies in comparison to 3D quadrupole ion traps of the same dimensions (Hager, 2002 Schwartz et al., 2002). In 2002, commercial LITs were introduced as either stand-alone mass spectrometers (Schwartz et al., 2002) or as part of a triple quadrupole mass spectrometer (Hager, 2002). 

The commercially available stand-alone LITs, marketed under the name LTQ, are made of four hyperbolic cross-sectional rods (Fig. 1.25). Since ions are trapped in an axial mode as opposed to central trapping on 3D ion traps, LTQs have been successfully coupled with Orbitrap and FTICR for achieving high-resolution capabilities (Peterman et al., 2005 Sanders et al., 2006) (Chapter 5). Functional improvements in 2D traps over 3D traps include 15 times increase in ion storage capacity, 3 times faster scanning, and over 50% improvement in detection efficiency and trapping efficiency. 

De vlieger, J., Krabbe, J. G., Commandeur, J. N. M., Vermeulen, N. P. E., Niessen, W. M. A., and Loftus, N. (2007). Characterisation of metabolites generated by mutant cytochromes P450 enzymes using a 3D ion trap-time of flight mass spectrometer. In Proceedings of the 55th ASMS Conference on Mass Spectrometry and Allied Topics. ASMS, Indianapolis, IN. 

A new generation of linear ion trap mass spectrometers has been developed and exhibits increased performance compared to traditional three-dimensional (3D) ion traps (Hopfgartner et al., 2003 Douglas et al., 2005). A further evolution of the triple-quadrupole family and ion trap class of instruments is the production of the hybrid triple-quadrupole/linear ion trap (QQQ/LIT) platform. Hybrid instruments of this nature allow for operation in space and not just in time when performing MS/MS analysis. This feature allows for increased performance compared to classical ion traps. A powerful combination possible on a hybrid LIT/QQQ instrument is the ability to use highly sensitive and selective precursor ion, constant neutral loss, and multi-MRM as a survey scan for dependent LIT MS/MS. Compared to a simple MS experiment, these comprehensive triple-quadrupole and LIT modes can be more complex to setup. 

Figure 3.3. Comparison of 3D ion trap (top) and triple-quadnipole/QTRAP (bottom) MS/MS spectra of dextromethorphan. The triple-quadrupole fragmentation occurs in the Q2 collision cell, which is also used in the hybrid QTRAP instrument, and yields much more fragmentation due to the higher energy collisions.

The quadrupole analyser is a device which uses the stability of the trajectories in oscillating electric fields to separate ions according to their m/z ratios. The 2D or 3D ion traps are based on the same principle. 

An ion trap is a device that uses an oscillating electric field to store ions. The ion trap works by using an RF quadrupolar field that traps ions in two or three dimensions. Therefore, ion traps can be classified into two types the 3D ion trap or the 2D ion trap. 

Historically, the first ion traps were 3D ion traps. They were made up of a circular electrode, with two ellipsoid caps on the top and the bottom that creates a 3D quadrupolar field. These traps were also named quadrupole ion traps (QITs). To avoid confusion, this term should not be used but should be replaced preferably with Paul ion trap. The acronym QUISTOR derived from quadrupole ion storage is also largely used but not recommended. 

Besides 3D ion traps, 2D ion traps have also been developed. They are based on a four-rod quadrupole ending in lenses that reflect ions forwards and backwards in that quadrupole. Therefore, in these 2D ion traps, which are also known as LITs, ions are confined in the radial dimension by means of a quadrupolar field and in the axial dimension by means of an electric field at the ends of the trap. 

Stability diagram along r and 2 respectively for a 3D ion trap. The iso/S lines for / = 0 (solid lines) and fSu = 1 (dotted lines) are drawn. The areas inside these limits correspond to stable trajectories for the considered coordinate. They correspond to imaginary solutions of the Mathieu equation. 

Stability areas along both r and z in a 3D ion trap. The common r and z stability area used in commercial ion traps is enlarged. This is displayed again in Figure 2.18. Redrawn and (modified) from March R.E. and Hughes R.J., Quadrupole Storage Mass Spectrometry, Wiley, New York, 1989, with permission. 

Typical stability diagram for a 3D ion trap. The value at = 1 along the qz axis is qz = 0.908. At the upper apex, = 0.149 998 and = 0.780 909. Drawn with data taken from March R.E and R.J. Hughes, Quadrupole Storage Mass Spectrometry, Chemical Analysis Vol. 102, Wiley Interscience, 1989. 

A 3D ion trap with an external ESI source (Finnigan LCQ). Ions produced by the ESI source are focused through a skimmer and octapole lenses to the ion trap. The gating lens is used to limit the number of ions injected in the trap, to avoid space charge effects (see further). 

Mass selective ejection of the ions in a radial direction occurs by applying an AC voltage between the two cut rods. As for the 3D ion trap, an AC frequency corresponding to qz = 0.88 is used. Ions of successively higher masses are brought to this qz value by increasing V. An ejection efficiency of about 50 % is achieved at 5 Th s scan rate [25]. 

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